1. Field of the Invention
The present invention relates to a filter circuit. More specifically, the present invention relates to a bandpass filter for use in an intermediate frequency amplifier in a radio receiver, for example.
2. Description of the Prior Art
In a radio receiver, for example, it has been proposed that the passband width of a filter circuit for an intermediate frequency signal be changed depending on the conditions of the broadcasting waves, the purposes of reception, and the like on the occasion of reception of the radio broadcasting. For example, a narrow bandpass filter is used to remove an interference noise. A narrow bandpass filter is selected also for tuning to a given broadcasting wave. On the other hand, a wide bandpass filter is used for the purpose of establishing a reception state for passage of an ample sideband to provide a good sound quality.
For the purpose of selectively switching the passband width of a filter circuit to a narrow passband width or a wide passband width, conventionally approaches like those shown in FIGS. 1 and 2 have been employed. More specifically, referring to FIG. 1, a narrow bandpass filter 31 and a wide bandpass filter 32 are disposed between a mixer 1 and an intermediate frequency amplifier 2 of a radio receiver, for example. By means of gang switches 41 and 42, either of the narrow bandpass filter 31 and the wide bandpass filter 32 is selectively connected between the mixer 1 and the intermediate frequency amplifier 2. However, the FIG. 1 structure requires two filters, i.e. the narrow bandpass filter 31 and the wide bandpass filter 32, which makes the cost high. Furthermore, since two filters are switched by means of the switches 41 and 42, it is necessary to make adjustment such that the center frequency f0 of both filters 31 and 32 is consistent.
The FIG. 2 example employs well known three-terminal ceramic filters 51 and 52. The ceramic filters 51 and 52 may be of a type employing a well-known expansion mode vibration. For example, divided electrodes are formed on one main surface of a ceramic plate 51a (52a) including PZT ceramic and a common electrode is formed on the other main surface. One of the divided electrodes is used as an input electrode 51in (52in), while the other is used as an output electrode 51out (52out), the common electrode 51c (52c) being connected to ground, thereby to complete a bandpass filter. A coupling capacitor 61 is inserted between the output electrode 51out of one ceramic filter 51 and the input electrode 52in of the other ceramic filter 52. A second coupling capacitor 62 is provided in parallel with the coupling capacitor 61 such that the second coupling capacitor 62 may be connected in parallel with or disconnected from the coupling capacitor 61 by means of switches 71 and 72. In the case of a narrow passband width, the switches 71 and 72 are opened or turned off, while in case of the width passband width the switches 71 and 72 are turned on or closed. Accordingly, the filter characteristic when the switches 71 and 72 are turned off is shown by the curve A in FIG. 3, while the filter characteristic when the switches 71 and 72 are turned on is shown by the curve B in FIG. 3. As seen from FIG. 3, by turning on or off the switches 71 and 72 the passband width can be selectively changed.
To change the overall passband width by changing the capacitance of a capacitor between two ceramic filters as shown in FIG. 2 is disclosed, for example, in the article entitled "A CERAMIC BANDPASS TRANSFORMER AND FILTER ELEMENT" presented by A. Lungo and F. Sauerland of Clevite Corporation in the IRE International Convention Record, PART 6, issued Mar. 20 to 23, 1961.
However, in the FIG. 2 example, the coupling capacitance is increased when the switches 71 and 72 are turned on, so that the passband width is extending only at the lower frequency side, as shown in FIG. 3, with the result that the center frequency is shifted toward a lower frequency region.